The present invention relates to cellular communication systems and more particularly to a new architecture for a cellular communication system to reduce co-channel interference and increase system capacity.
In cellular communication systems, frequency reuse plans allow the same frequency to be used more than once within the system. Thus in contrast with broadcasting, cellular systems divide a service region into smaller cells that transmit different signals for different users. The available frequencies are divided into frequency groups and each cell is assigned at least one frequency group to use for communications with mobile terminals within that cell.
To avoid the problem of co-channel interference, cells assigned the same frequency group, known as co-channel cells, are spatially separated so that the mobile terminal operating within a cell receives the desired signal at a higher level than any potential interfering signal from co-channel cells. Cells operating at different frequencies are placed between any two co-channel cells. In general, the power of any interfering signal diminishes with increasing distance between interfering users. A frequency group can be reused if the interference level is reduced sufficiently by separation between the co-channel calls. The interference level is measured by the carrier power to interference power ratio, C/I. The C/I ratio is the primary criteria used in designing frequency reuse plans.
From the foregoing, it should be apparent that the number of times a given frequency can be reused in a system is related to the separation distance or reuse distance between any two co-channel cells, and the radius of the cells. Current systems serve a cell of given radius using a large centrally-located tower. Unfortunately with this method of covering the service area, the spill-over of interference into neighboring cells diminishes only slowly with distance. Therefore, there is an interest in a method of coverage which gives more rapid unwanted signal attenuation outside the service area.